1. Field of the Invention
The invention relates to a system for the automatic correction of the color balance of signals that determine the color and luminosity of a color television picture.
2. Description of the Prior Art
From the German Offenlegungsschrift No. 2,237,784 it is known to modify the chromaticity of a color television picture by means of a color corrector. The known color corrector comprises operating elements by means of which the chromaticity is manually adjustable within certain luminosity ranges of the color television picture according to the subjective perception of an observer.
Furthermore, from the German Offenlegunsschrift No. 2,141,685 it is known to carry out automatically color corrections of a color movie film in a pre-programmed manner. Such a pre-programmed color correction requires that sufficient time be available for a test run of the color movie. During the test run the color defects subjectively determined by an observer are stored scene upon scene. During the broadcast, the individual scenes are corrected in correspondence to the stored color defect values. In the case of certain color television broadcasts, e.g., news broadcasts with actual interpolations from other broadcasting stations, the prerun necessary for the preprogramming of a color correction is not possible. Such actual interpolations are broadcast without having been watched previously. Since furthermore the scenes of news broadcasts change very frequently, the video engineer finds it impossible to eliminate observed color defects while the individual scenes are being presented on the screen.
To overcome this disadvantage, an automatic color correction system has already been proposed wherein the color balance of the red and blue primary signals of a color signal source relative to the green primary signal of said source is automatically established. The balancing process takes place in this system when the amplitudes of the luminance signals drop below 7% or rise above 95% of the maximum luminance signal value, i.e., when they correspond to the black or white values of the luminance signal. This balance criterion is based on the assumption that the voltage level of the color signal becomes zero in black or white picture portions. The presence of a color signal in black or white picture portions, on the other hand, shows that a discoloration exists, and therefore no color balance between the primary signals exists. Such a system, however, presents undesired effects upon blue and skin-colored picture portions since a dark blue or a bright yellow likewise appears as black or white, respectively. Although no discoloration exists in the color television picture, the aforementioned balance criterion alone would lead to further color defects. For this reason, it has further been proposed to cut out the balancing process in dark blue and bright yellow hues. The hue recognizing circuits necessary for this purpose, however, are very expensive. Moreover, the balancing range, already narrowed down to less than 7% ang greater than 95% of the luminance signal, is even further narrowed down by this device. Besides, it becomes obvious that a satisfactory color rendition also requires a correction of the gray values (gamma correction). To carry out this correction automatically has hitherto been impossible since sufficient criteria were lacking.